GATE 2023 AG – Question 41
A six-hour S-hydrograph has ordinates0,30,90,180,252,306,342,360,360 m³/s at 0,6,12,18,24,30,36,42,48 hours. The φ-index is .25 cm/hour and base flow 10.5 m³/s. A 45 mm storm falls during the first six hours. Find flood peak in m³/s.
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Correct answer: (C) 280.5
Explanation
Subtract the S curve shifted by six hours to recover six-hour unit-hydrograph ordinates:0,30,60,90,72,54,36,18,0. Unit-hydrograph peak is 90 m³/s per cm excess. Storm excess is 4.5−.25(6)=3 cm. Direct peak is 3(90)=270 m³/s; adding base flow gives **280.5 m³/s, C**.